Disk files in which information is stored on concentric tracks on one or more recording disks are well known. Information is written on and/or read from a disk, while it is rotating, by a transducing head supported adjacent the disk surface. Magnetic disk files are the most common, particularly for data processing applications. Such files employ an electromagnetic head in conjunction with a magnetisable recording disk. However, other transducing effects, e.g. optical and electrostatic, are known and are of increasing importance in the video recording industry.
Irrespective of the transducing effect or the precise application, at the higher track densities the disk file must be provided with position reference information which is employed by a head positioning servo system to position and maintain the head precisely over a selected track of the disk. The operation of maintaining the head over a desired track is known as "track-following", whereas that of moving the head between tracks is known as "track-accessing". It is with improvements in the track-following operation that the present invention is concerned.
In some disk files, the position reference information is provided remotely from the disk surface on which the information or data to be process is stored, e.g. on a dedicated servo disk or surface, and is available continuously. It is also known to provide position reference information in sectors, known as "servo sectors", on the information storage surface. These servo sectors are interspersed with "data sectors" containing the stored information and provide accurately registered position reference information on a sampled basis as the rotates disk.
Whichever form the position reference information takes, it defines nominally circular tracks on the disk which are intended to be concentric with the axis of rotation of the disk medium. For various reasons neither the circularity nor the concentricity of the tracks so defined may be perfect. In the case of removable disks, the rotation axes of different drive mechanisms may not all align concentrically with the tracks on a particular disk, when mounted for rotation in the drive mechanism. Also, defects in the record medium itself may result in errors in the written position reference information. Finally, especially in the case of flexible disk media, the shape of the disk itself may change with age as a result of temperature and humidity cycling.
The above factors together with mechanical runout (radial displacement) of the disk when it is rotated can all, of course, be accommodated, to some extent, by the operation of the track-following servo system. However, there is a limit to the responsiveness (gain) of such servo systems if instabilities at certain resonant frequencies are to be avoided. This means that, where significant eccentricity exists, the servo system is not capable of following deviations of the tracks exactly and errors in reading and writing data may result. In any event, the achievable track density is restricted which, in turn, imposes a limit on the information storage capacity of the disks.
As employed herein, the term "eccentricity" is intended to encompass both non-concentricity of a circular track with the axis of rotation of the disk and also non-circularity, e.g. ovality, of the track, whether concentric with the axis of rotation or not.
Where the cause of error is permanent and fixed, e.g. a defect in the magnetisable record medium, it has been proposed to include servo correction data in the position reference information at the time of manufacture. One such scheme is described in our U.S. Pat. No. 4,412,165. In that application, a servo correction byte is associated with each servo sample to correct defects in faulty servo samples.
In other cases, such as gradual distortion of disks with age or the remounting of disks on different drive apparatus, it has been recognised in the art that a predictive correction can be applied directly to the servo system, providing the error is repeatable during operation.
One such proposal is described in an article by J. P. Mantey entitled "Disk Runout Accommodation" (IBM Technical Disclosure Bulletin, Vol. 21, No. 7, December 1978). In the system proposed in that article, an estimate of runout is used in combination with a closed loop track-following servo system. The runout estimate is computed on the basis of a mathematical model of the dynamic behaviour of the system.
In U.S. Pat. No. 4,135,217, a hybrid servo system for positioning the heads of a disk file is described. Track-following is effected by means of sector servo samples, interspersed with the data, which are detected by a read/write head. The head is moved by means of an actuator (motor and carriage) which has its own position transducer of the optical grating type. The access motion is primarily controlled by reference to this external transducer. However, to overcome some of the above-mentioned disadvantages, the output of the external transducer is also stored while the head is track-following to provide an indication of the "wobble", or repeatable error in track shape. It is assumed that, when the head is fine-positioned (i.e. track-following) during an initial calibration period, the variation in the external transducer output will indicate the deviation of the track being followed from concentricity. This information, which is stored in digital form for each sector, is used to predict subsequent fine positioning movement of the head in addition to the normal closed loop feedback control of head position error.
Finally, to complete the review of the prior art, reference is made to the following articles: "Self-calibrating disk storage apparatus" by D. E. Griffiths and H. E. Van Winkle (IBM Technical Disclosure Bulletin, Vol. 19, No. 6, November 1976, p. 1991); "Track-locating and following apparatus for a flexible disk file" by R. C. Jahnke (IBM Technical Disclosure Bulletin, Vol. 23, No. 2, July 1980, p. 738). These articles both describe open loop rather than track-following head positioning systems for disk files. However, both employ a limited amount of servo information which can be used to generate a representation of track shape. This representation is stored and used subsequently to modify the open loop positioning control of the heads as the disk rotates.